1. Field of the Invention
The invention relates to a polyethylene terephthalate fiber of improved dyeability and to a process for the preparation thereof. More particularly, the invention relates to an easily dyeable polyethylene terephthalate fiber which can be dyed at 100.degree. C., i.e., under normal pressure, without the use of a carrier after being false twisted and to a process for preparing the fiber by high-speed spinning at a spinning speed of not less than 7,000 m/min.
2. Description of the Prior Art
Polyethylene terephthalate fibers are widely used in the garment industry. They are, however, poor in dyeability, and, thus, it is necessary to dye them by using a high-pressure dyeing machine at a high temperature of about 130.degree. C. and under a high pressure or by using a carrier of an organic solvent. High-temperature and high-pressure dyeing has disadvantages in that much energy is necessary and in that the fibers cannot be substantially used in combination with other fibers, such as wool, acrylic fibers, or polyurethane fibers, which fibers are degraded during high-temperature and high-pressure dyeing. On the other hand, carrier dyeing has disadvantages in that, due to the use of an organic solvent as the carrier, the process is complicated, the odor of the used solvent remains on the product, and treatment of the waste liquor is difficult.
Therefore, it is very advantageous if a polyethylene terephthalate fiber which can be dyed at a temperature lower than 130.degree. C. can be obtained. Particularly, if it is possible to dye a polyethylene terephthalate fiber at a temperature not higher than 100.degree. C., i.e., under normal pressure, the following advantages can be attained: energy can be saved, the use of a carrier is unnecessary, and excellent new textiles, such as mixed knitted or woven fabrics, can be obtained since the polyethylene terephthalate fibers can be used in combination with other fibers such as wool, acrylic fibers, or polyurethane fibers which are degraded by dyeing at 130.degree. C. Therefore, the utility of the polyethylene terephthalate fibers can be increased.
Such an easily dyeable polyethylene terephthalate fiber has another advantage in that the use of an expensive high-pressure dyeing machine, the control of which is complicated, is unnecessary, i.e., an inexpensive and simple dyeing machine such as a jigger can be used.
A method in which a third component, such as a compound having a metal sulfonate group, is copolymerized with polyethylene terephthalate is known as a method for improving the dyeability of a polyethylene terephthalate fiber. However, in this method, the thermal and mechanical properties, such as the melting point and strength, inherent to polyethlene terephthalate may be deteriorated. In addition, it is still impossible to dye the resultant fiber in combination with wool, an acrylic fiber, or a polyurethane fiber without the use of a carrier. Further, such a copolymerized polyethylene terephthalate may often have a poor light fastness when dyed.
Japanese Examined Patent Publication (Kokoku) No. 35-3104 discloses that highly oriented filaments having practical, satisfactory properties can be obtained by high-speed spinning, in which melt-spun polyethylene terephthalate filaments are taken up at a speed of not less than 4,000 m/min even if the filaments are not subjected to drawing. U.S. Pat. Nos. 4,156,071, 4,134,882, and 4,195,051 and Seni Gakkaishi, 37, No. 4, pages T135 to T142 (1981) disclose that polyethylene terephthalate fibers obtained by high-speed spinning at not less than 4,000 m/min have a higher dyeability than do polyethylene terephthalate fibers obtained by a conventional process in which polyethylene terephthalate is melt spun at a low speed and the resultant filaments are then subjected to drawing.
The polyethylene terephthalate fiber disclosed in U.S. Pat. No. 4,156,071 has a high dyeability since it is spun at a speed of about 4,000 m/min. However, the fiber has a serious practical disadvantage in that the fiber is elongated by a relatively low load at the weaving or knitting step due to the low first yield stress, and, thus, a fabric obtained from the fiber may often have uneven dyeing or a poor quality. Also, the fiber has an initial modulus of about 50 g/d, which is approximately equal to that of a cellulose acetate fiber and, thus, does not maintain excellent hands inherent to a conventional polyethylene terephthalate fiber.
The polyethylene terephthalate fiber disclosed in U.S. Pat. No. 4,134,882 has a long period of not less than 300 .ANG., a low distribution of birefringence across the transverse cross section of a filament, and a high dyeability. This fiber may be prepared by a process disclosed in U.S. Pat. No. 4,195,051, in which process a spinneret having nozzles of a length diameter ratio larger than usual is used and spun filaments are taken up at a speed of not less than 5,200 yards/min (i.e., 4,700 m/min). In these two U.S. patents, examples are given in which spinning is carried out at a speed of from 4,950 m/min to 7,200 m/min. However, in the disclosed process, the higher the spinning speed, the greater the air drag, with the result that yarn breakage may often occur. In order to avoid this problem, it is necessary to decrease the fineness of the filaments to be spun (i.e., decrease the surface area per unit weight) as the spinning speed is increased. It has conventionally been impossible to obtain a polyethylene terephthalate filament fiber having a fineness of not more than 4 deniers, i.e., a surface area per unit weight of not less than 1,400 cm.sup.2 /g, which is suitable for making garments at a spinning speed of not less than 7,000 m/min. In addition, the polyethylene terephthalate fiber obtained by this process cannot have a dyeability enabling it to be dyed under normal pressure even after the fiber is false twisted.
By the process disclosed in Seni Gakkaishi, 37, No. 4, pages T135 to T142 (1981), in which process polyethylene terephthalate is spun at a high speed while cooling the as-spun filaments with cooling air of -2.degree. C. immediately after extrusion from the spinneret, a polyethylene terephthalate filament fiber of a fineness of not less than 5.8 deniers (i.e., a surface area per unit weight of not more than 1190 cm.sup.2 /g) can be obtained at a spinning speed of from 7,000 m/min to 9,000 m/min. This publication further discloses that the polyethylene terephthalate fiber obtained at a spinning speed of not less than 7,000 m/min has a high dyeability which is further improved as the spinning speed increases. However, the fiber cannot have a dyeability enabling it to be dyed under normal pressure even after false twisting is carried out.
As was mentioned above, in known high-speed spinning processes, it is impossible to spin a polyethylene terephthalate filament fiber having a fineness of not more than about 4 deniers at a speed of not less than 7,000 m/min, and, thus, a polyethylene terephthalate fiber which can be dyed under normal pressure after false twisting cannot be obtained.
Japanese Unexamined Patent Publication (Kokai) No. 51-7216 discloses a process for preparing a polyester fiber at a spinning speed of from 2,000 m/min to 5,000 m/min, in which process the as-spun filaments are bundled at a point not more than 25 cm beneath the hardening point (i.e., the point of completion of fining) of the filaments. However, even if the process as such is applied in high-speed spinning of not less than 7,000 m/min, spinning is impossible due to the frequent occurrence of yarn breakage. This publication is completely silent concerning means for making possible spinning at a speed of not less than 7,000 m/min.